Trouble setting cookies - Apache 1.3.27, PHP 4.3.1, Win2K, MySQL 4.0.13-max-debug, PHPLib 7.4-pre2

Using Mozilla 1.3 (HTTP Mozilla/5.0) and IE 5.50 SP 2, session4.inc does not actually set a session cookie.  At one point before re-installing Apache 1.3.27, I was able to set a cookie in both browsers, but something has broken since and I have not been able to recall which of my configuration actions was responsible.  I cross-compared my old httpd.conf with my new one to try to ensure the same environment on the webserver as was successful previously.

PHP interprets all PHP scripts properly and I'm getting no PHP errors in my log file nor Apache errors in my error.log.

I mainly use Mozilla, which is set to prompt me if a cookie wants to be set, and I have double-checked to ensure that it is not set to block cookies from localhost.

I run the ZoneLabs Pro firewall but have unloaded it completely to test localhost scripts, in case it was interfering with cookies (but it hasn't interfered with any others when loaded and is set to allow localhost cookies when running).  But no joy.

Here is the basic PHP test script I used to try setting a test cookie (first lines in script) - without calling PHPLib's session (or any other) classes:
<?php
setcookie("osp", "hello", time()+3600, "/", ".localhost", 0);
?>
<html>
[...]
</html>

Here are HTTP-Headers from phpinfo(), which result from using PHPLib's prepend.php (from phplib 7.4-pre2) for db virtualization including session setup and have double-checked all PHPLib manual instructions (the only test that doesn't work is incrementing the SID with test.php3 - I have register_globals=Off in php.ini).  Note the weird expiration date on the Set-Cookie instruction in HTTP Response (PHPLib simply uses (date()*60)+43200 to set the cookie lifetime), which should expire in 30 days from cookie set date:

HTTP Request     GET /test.php3 HTTP/1.1
Accept     text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,video/x-mng,image/png,image/jpeg,image/gif;q=0.2,*/*;q=0.1
Accept-Charset     ISO-8859-1,utf-8;q=0.7,*;q=0.7
Accept-Encoding     gzip,deflate,compress;q=0.9
Accept-Language     en-us,en;q=0.5
Cache-Control     max-age=0
Connection     keep-alive
Host     localhost
Keep-Alive     300
Referer     http://localhost/index.php3
User-Agent     Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.3) Gecko/20030312
User-Agent     Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0; T312461)

HTTP Response Headers
Set-Cookie     osp=036aafb8e93397edd3733011430cc331; expires=Sun, 09-Nov-36 03:59:51 GMT; path=/; domain=localhost
Expires     Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control     no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma     no-cache
Keep-Alive     timeout=15, max=100
Connection     Keep-Alive
Transfer-Encoding     chunked
Content-Type     text/html

Here are my php.ini [Sessions] directives (all of these show up in phpinfo()'s Sessions block):
session.save_handler=files
session.save_path="C:\phpdev\php\temp"
session.use_cookies=1
session.use_only_cookies=False
session.name=PHPSESSID
session.auto_start=0
session.cookie_lifetime=0 ; 2592000
session.cookie_path=/
session.cookie_domain=localhost
session.cookie_secure=False
session.serialize_handler=php
session.gc_probability=5
session.gc_maxlifetime=1440
session.referer_check=
session.entropy_length=0
session.entropy_file=
session.cache_limiter=nocache
session.cache_expire=1
session.use_trans_sid=0
url_rewriter.tags="a=href,area=href,frame=src,input=src,fieldset=" ; form=fakeentry removed for XHTML compatibility
session.bug_compat_42 = 0
session.bug_compat_warn = 1

Here is my <IfModule mod_mime.c> from httpd.conf (ServerName localhost):
LoadModule php4_module "c:/phpdev/php/sapi/php4apache.dll"
# AddModule mod_php4.c [Note: apache -t complained - module not found - when I uncommented this]
# ScriptAlias /php/ "c:/phpdev/php/"
AddType application/x-httpd-php .php4
AddType application/x-httpd-php .php
AddType application/x-httpd-php .php3
AddType application/x-httpd-php .inc
AddType application/x-httpd-php .fire
AddType application/x-httpd-php .osp
AddType application/x-httpd-php .htm
AddType application/x-httpd-php-source .phps
# Action application/x-httpd-php "/php/php.exe"

Can't think of anything else - thanks in advance,
Stephen